Better Deburring in a Continuous Vibrator

Ask an Expert From: Products Finishing, from The Markee Corp.

Posted on: 4/1/2010

What disadvantages, other than wear rate, will result from using faster cutting media?

 Q. We’re deburring 12-gauge steel stampings in a continuous vibratory finisher. The stampings average 2 × 6 inches. The burrs are around the outside edges, and around the four 3/4-inch diameter holes. Our machine has 30ft3 capacity. The tub is a 12-foot-long trough, and the parts stay in the process for 10-12 min. Our media are medium-cutting, 45°-angle-cut cylinders, size 5/8 × 11/4 inch. Deburring around the edges is adequate, but one customer wants better deburring around the holes. Once, we double-passed this customer’s parts, and he really liked the results. He now wants us to do that all the time. The problem is not only who should pay for this, but that it reduces our deburring capacity. It may require a second, or at least a larger machine, and we currently have a moratorium on capital spending. 

Our machine has variable speed control; we tried higher RPM, but that did not accomplish the desired deburring. Our vendor for mass finishing supplies and equipment has suggested faster cutting media. This, he says, may only increase our media cost by about 30–40%. It will be less costly than double passing. He has also suggested a bowl vibrator with an internal separator just for this customer’s parts. We can then process as long as necessary without changing anything else. This sounds good, but once again, additional equipment is not an option. This raises several questions: Can I increase the time cycle in the continuous vibrator only for this customer’s parts? What disadvantages, other than wear rate, will result from using faster cutting media?  J.O.
 
 
 
A. There is more to solving your problem than the suggestions made by your vendor. Time cycles are somewhat adjustable in continuous, straight-through machines. Some were designed with variable adjustments. Design features offered include: raising and lowering one end of the machine; adjusting the discharge spout height; installing an adjustable dam at the exit end; changing the thrust angle by adjusting the eccentric shaft alignment with the bowl; variable speed drives; and, diverting a portion of the media to a holding device. 
 
These features all work to some degree because all these machines are gravity and/or displacement dependent. That is, as you pour something in one end, something else will pour out of the other end. The entire mass acts very much like a viscous liquid so long as the motor is running. The exit end usually has some sort of dam, or spout, and the level of that device controls the level in the mass. I infer that the only design option available to you is the variable speed. In most through-feed machines, the eccentric shaft is lower at the exit end, encouraging a forward travel of the parts. So, when you increased the RPM, you also increased the amplitude, raised the level of the media, and thrust the parts forward more rapidly. This more aggressive action is, therefore, partially offset by a shorter time cycle. (In these machines, as with most bowl machines, vibration frequencies around 1,200 VPM seem to be optimum.)
 
There’s an operating variable you can apply to accomplish different time cycles. As you raise or lower the mass level, the time cycle will get shorter, or longer, accordingly. Finding the level that gives the right time cycle is worth the effort, and you’ll learn a lot about how your machine should be operated. Once you have the desired level and time cycle, maintain that condition with a constant part feed rate, plus regular, small additions of media. 
 
You can lower the mass level by reducing the parts, the media, or both. I’m sure you want to maintain productive capacity, so I’m recommending a procedure that decreases the ratio of media to parts. In most continuous machines this ratio is 7-10/1 media/parts. With your parts, this ratio can safely be changed to 3-4/1. My recommendation will maintain productivity while giving the desired longer time cycle. The trade-off will be more parts in the machine with a slight possibility of part-on-part damage. This should not be a problem if you don’t exceed 20-min cycles at the given parts feed rate.
Here’s a method to quickly determine the time cycle: Your machine is 12 ft long (144 inches); a 10 min time cycle means the mass is traveling forward at 14.4 inches/min. Your target is half of that, or about 7 inches/min. To measure the travel rate attach a 3-inch PVC pipe cap to a part. It pops to the surface on each roll of the mass. Mark a spot where it surfaces, and time it for a few revolutions, measuring the distance traveled. For example, say three revolutions take 75 sec and the part travels 15 inches. That’s 12 inches/min. Twelve feet traveled at 12 inches/min is a time cycle of 12 min.
 
Keeping the same parts feed rate, remove about two boxes of media. Wait at least 20 min and then time the forward travel again. Based on this change, you can make a good guess as to how much more media to remove for the desired time cycle. Make that change and time it again. You may only need one small adjustment after that. If you can accept the deburring and finish results, you will have the longer time cycle without losing production.
Considering all of the above, you can understand why your vendor recommended a bowl-style machine. These machines can process for any time cycle. When the job is done, whether in 10 minutes or an hour, the parts are removed using the internal separation device. If that were the style of your existing machine, you would not be having the current problem.
 
There are several grades of media that will cut faster than your current selection. That is one of your options. Faster cutting media has traditionally been faster wearing. The method has been to increase the percentage of abrasive in the media, and possibly also to change to a more aggressive abrasive, specifically silicon carbide. In addition, the ceramic bond may be made softer—and faster wearing—to expose more of the abrasive to the surface while processing. In an extreme example, the cutting rate may be doubled while the wear rate is increased to as much as 400%. These higher wear rates increase the volume of media dust to be separated and discharged. Every pound of media consumed is another pound of waste impacting the environment. But, fortunately, there are some better answers with today’s 
media. 
 
The industry is coming to understand that heavy media cuts faster than light media, all other things being equal. The result is that much heavier media products have become available. Just a few years ago, the average density of cutting media was 85 lbs/ft3. Then, tougher, longer lasting bonds became available weighing between 105 and 115 lbs/ft3. With finer abrasives, these can still cut equal to previous fast cutting bonds, but with lower wear rates. The trend is now going to even heavier bonds, more than 125 lbs/ft3. These media are more expensive, but justify their existence with faster cutting and lower wear rates. They reduce the disposal volumes accordingly, making it environmentally more acceptable. I expect to see this trend continuing, with the average ceramic bond in the future weighing more than 100 lbs/ft3.
Now, to complete my answer to your questions: A faster cutting media may get the job done in a single pass. And, you have seen that you can vary the time cycle. If you do this with the new heavy bonds, you will not have the cost penalty of lightweight, fast cutting products. Explore these options with your current vendor. Be aware that heavier media will require re-adjustment of the eccentric weights. 
 
The other answer is first thing to be explored: Change the geometry of your media. You may stay with the current bond, or try the new bonds, but the shape must go!
 
The least effective shape for deburring ID’s is the cylinder. It is also the most commonly recommended by many vendors. Every few years I write about media geometries in this column, and I can refer you to several past articles for detailed discussions. Please, get rid of the cylinders, and your problem will be solved. The questions of single pass versus multiple passes, increasing the time cycles, buying new equipment, having higher media consumption—all of those will go away.
 
What media shape should you use? My first suggestion without seeing the parts is a 5/8 × 5/8-inch, 45° angle-cut triangle. My second suggestion is a 5/8-inch WEJ, also known by such names as “tri-cut cylinder”, and “V-cut cylinder.” For added benefit, try the new heavy cutting media in these shapes.



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